Chloroacetamides are known herbicides. More particularly, they are plant growth inhibitor herbicides which primarily inhibit growth of roots and shoots of seedlings. Examples of chloroacetamide herbicides are alachlor, metolachlor, acetochlor, metazachlor, diethatyl, propachlor and thiophenamines. An example of a known thiophenamine plant growth inhibitor herbicide is dimethenamid, whose chemical name is 2-chloro-N-(2,4-dimethyl-3-thienyl)-N-(2-methoxy-1-methylethyl)-acetamide. Processes for its production, herbicidal compositions containing it and its use as a herbicide are described in U.S. Pat. No. 4,666,502, the contents of which are incorporated herein by reference. Dimethenamid consists of 4 stereoisomers as diastereomeric mixtures (1S, aRS (known as S-dimethenamid) and 1R, aRS (known as R-dimethenamid)) and as a racemic mixture (1RS, aRS). References herein to chloroacetamides, including dimethenamid, refer to their various forms, including their various stereoisomers, unless stated otherwise.
One commercial dimethenamid product is available under the registered trademark FRONTIER.RTM. (BASF AG, Germany), with either G.0 lb/gal. or 7.5 lb/gal. dimethenamid, along with other inert ingredients, such as petroleum distillates, xylene or xylene range aromatic solvents.
While the use of, chloroacetamides, including dimethenamid, as growth inhibitor herbicides is known in the art, one drawback to their commercial use is their precipitation point--the temperature, at about standard atmospheric pressure, at which liquid chloroactamides begin to solidify to form a solid precipitant. The racemic mixture of dimethenamid has a precipitation point of about 20.degree. C. -22.degree. C. As a result of this property, these commercial products tend to precipitate from liquid formulations at temperatures which are common in commercial use of herbicides.
For example, the FRONTIER.RTM. product comprising 7.5 pounds of dimethenamid per gallon, tends to form a solid precipitant at temperatures of 12-13.degree. C. and below. The temperatures experienced by these formulations during normal distribution and field application commonly drop to temperatures well below 12-13.degree. C., thus resulting in formation of dimethenamid precipitant. This is problematic to commercial users because, among other things, precipitation inhibits the users' ability to uniformly apply the herbicide to crops. Thus, commercial users typically must heat the dimethenamid products prior to use, which can be costly and time consuming. Alternatively, manufactures of dimethenamid products are required to rotate stock of dimethenamid in heated storage with unused dimethenamid at the commercial users' facilities that has been exposed to temperatures below 120.degree.-13.degree. C.
It is well known that the temperature at which a dissolved liquid freezes, or precipitates, can be lowered by decreasing the mole fraction of the solute in solution of the liquid solvent. The extent to which the precipitation temperature is affected is generally directly proportional to the extent to which the mole fraction of the solvent has been decreased.
If a solution is an "ideal" solution, the extent to which the precipitation temperature decreases by addition of a solute is not affected by the composition of the solvent or solute, and a curve made by plotting precipitation temperature versus concentration will not vary when different compounds are used to dilute the liquid. The term "ideal solution" refers to a solution in which little or no specific molecular interaction occurs between its components. An "ideal solution" conforms with Raoult's law.
Thus, one theoretical alternative approach to avoiding the need to heat chloroactetamide herbicides prior to use is to significantly lower the mole fraction of (i.e., dilute) the herbicide in solution. One preferred diluent known as gamma butyrolactone can be so used to lower the melting point of dimethenamid to minus twenty degrees celsius, but in order to do so, the dimethenamid in the solution must be diluted to twenty mole percent (forty five percent by weight).
However, by significantly diluting the herbicide, its effectiveness is reduced. Furthermore, significant dilution of the herbicide results in a significant increase in the amount of total product required to achieve the desired herbicidal result. This not only results in greater cost to the user based on the amount of product purchased, but also increases significantly the costs of shipping, handling and applying the product.
Extensive experimentation was conducted in attempting to lower the precipitation temperature of the chloroacetamide herbicide, dimethenamid, by combining it in solution with various substances, and lowering the temperature of the solutions incrementally while observing them for solid precipitant formation. The dimethenamid precipitation temperature for each solution was determined as the temperature, at about standard atmospheric pressure, at which the solutions yielded at least a trace of solid dimethenamid precipitant. The term "trace" is used herein to mean an amount of solid precipitant that can be detected visibly without the aid of magnification, but which cannot be measured quantitatively without the aid of magnification. If the amount of solid precipitant can be measured visibly without the aid of magnification, then it is considered to be more than a trace.
It is understood that most substances form ideal, or nearly ideal, solutions with dimethenamid, and therefore, that the melting point of dimethenamid is not depressed substantially without significant dilution of the dimethenamid. Although some compounds have a minor effect on precipitation temperature, the deviation from ideality with these substances is not significant enough to be useful, and the substances are not acceptable in commercial herbicide formulations.
Accordingly, no method of inhibiting solid precipitant growth in chloroacetamide solutions at conventional shipping, storage and application temperatures, other than unacceptable dilution, is currently available. Therefore, commercial users of chloroacetamide herbicides, such as dimethenamid, have been unable to use such liquid products, substantially free of solid precipitant, if such products have been shipped or stored at temperatures substantially below 12.degree.-13.degree. C., without having to heat the product to melt, or re-dissolve, the solid precipitant therein. Because known diluents can depress the precipitation point only by substantially diluting the herbicide, the users' only alternatives in those conditions have been to either use such products containing solid precipitant therein or to employ the costly and time consuming step of heating the products to melt, or re-dissolve, the solid precipitant.